Both resistive ribbon thermal transfer printing and electroerosion printing are known in the art for providing high resolution, good quality printing, especially of the type that is used in computer terminals and typewriters. Resistive ribbon thermal transfer printing is a type of thermal transfer printing in which a thin ribbon is used. The ribbon is generally comprised of either three or four layers, including a support layer a layer of fusible ink that is brought into contact with the receiving medium (such as paper), and a layer of electrically resistive material. In a variation, the resistive layer is thick enough to be the support layer, so that a separate support layer is not needed. A thin electrically conductive layer is also optionally provided to serve as a current return.
In order to transfer ink from the fusible ink layer to the receiving medium, the layer of ink is brought into contact with the receiving surface. The ribbon is also contacted by an electrical power supply and selectively contacted by a thin printing stylus at those points opposite the receiving surface (paper) where it is desired to print. When current is applied via the thin printing stylus, it travels through the resistive layer and causes localized resistive heating which in turn melts a small volume of ink in the fusible ink layer. This melted ink is then transferred to the receiving medium to produce printing. Resistive ribbon thermal transfer printing is described in U.S. Pat. Nos. 3,744,611; 4,309,117; 4,400,100; 4,491,431; and 4,491.432.
The materials used in resistive printing ribbons are well known in the art. For example, the resistive layer is commonly a carbon or graphite-filled polymer, such as polycarbonate. The thin current return layer is a metal, such as Al. The thermally fusible inks are comprised of various resins having a colorant therein, and typically melt at about 100 degrees C. Printing currents of approximately 20-30 mA are used in the present, commercially available printers, such as those sold by IBM Corporation under the name QUIETWRITER.TM..
The QUIETWRITER.TM. typewriter employs a correction feature whereby "printed" ink is transferred from printed paper to ribbon. An unused section of ribbon is positioned over the image to be corrected. The printhead is then swept over the ribbon as if to print a block image over the character to be corrected. The energy applied to the block is lower than that used for printing but is sufficiently high to produce a bond to the image to be corrected. A correction roller is introduced to delay peeling of ribbon from the page. This allows formation of a stronger bond between the ribbon and the image to be corrected before peeling occurs.
Electroerosion printing is also well known in the art, as exemplified by U.S. Pat. No. 3,786,518; 3,861,952; 4,339,758; and 4,086,853. Electroerosion printing is known as a technique which is suitable to make direct offset masters and direct negatives. Generally, the electroerosion recording medium is comprised of a support layer and a thin conductive layer. The support layer can be, for example, paper, polyesters such as Mylar.TM., etc., while the thin conductive layer is a metal, such as Al. In order to print portions of the thin Al layer are removed by an electric arc. To do so, a printing head comprising multiple styli, typically tungsten wire styli of diameters 0.3-0.5 mil, is swept across the electroerosion medium while maintaining good electrical contact between the styli tips and the aluminum layer. When an area is to be printed, a pulse is applied to the appropriate styli at the correct time resulting in an arc between the energized styli and the aluminum layer. This arc is hot enough to cause local removal of the aluminum by disintegration, e.g., vaporization.
Practical electroerosion media require a base layer between the supporting substrate and the thin metal layer, as well as an overlayer on the thin metal layer. The base layer and the overlayer are used to prevent scratching of the aluminum layer in areas where no arc is applied, and to minimize head wear and fouling. Typically, the base layer is a hard layer consisting of hard particles embedded in a suitable binder, such as silica in a cross-linked cellulosic binder. The overlayer is typically a lubricating, protective overlayer comprised of a polymer including a solid lubricant, such as graphite in a cellulosic binder.
Depending upon the properties of the various layers in the electroerosion medium, direct offset masters and direct negatives can be formed. For example, a direct negative can be comprised of a transparent polymer support layer and a thin aluminum layer directly deposited on the support layer. After electroerosion writing the Al layer is patterned. Since the Al layer is reflective to light while the substrate is transparent, the electroerosion writing has produced the required light opaque and light transparent regions needed to make a negative. The electroeroded negative can be used in a plate-making machine of the type used to make a "master", such as that used in offset photolithography. The master would be made by contact printing using the electroeroded negative.
A direct master can be easily made by electroerosion in order to simplify the process by which masters or plates, are made in conventional offset lithography shops. In such a structure the electroerosion recording medium is typically comprised of the support layer, a base layer which is hydrophobic, an Al layer and an optional overlayer. When the Al layer is electroeroded and the overlayer removed, regions of the Al layer (unwritten areas) and the base layer (written areas) will be exposed. Since the Al layer is hydrophilic, the unwritten areas having Al will attract water but repel organic inks. The written areas of the recording medium, being comprised of the hydrophobic base layer, will repel water but will accept organic based inks. A direct master is thereby produced since the information to be printed has been successfully mapped onto the master in terms of surface affinity to water and ink.
If the problem of scratching of the Al layer in undesired areas were not present, the substrate-Al layer combination could itself be used for direct master and direct negative applications. For a direct negative, a clear polymer sheet, typically polyester, can be used as the substrate. Since this is transparent to light while the Al is reflective, a direct negative would be obtained. Also, since the Al is hydrophilic and the polyester substrate is hydrophobic, a direct master would also be created in principle.
Heretofore, electroerosion has been used to provide lithographic printing masters and direct negatives, but the lifetimes of these masters and negatives in actual use is not as extensive as when the traditional printing plates are made using various chemical treatments to prepare a photosensitized plate.
Generally, the prior art has not provided a technique for creating direct negatives using equipment which is energy efficient and suitable for processing using commercially available apparatus.